Objective: The study of the immunocompetence of patients undergoing radical radiotherapy. The correlation of the immunological status with tumor radiosensitivity and prognosis. Approach: Delayed hypersensitivity, as determined by skin reactivity to 2-4 dinitrochlorobenzene (DNCB) was tested in 112 patients prior to undergoing curative radiotherapy. Of the strong DNCB reactors, 84% had an excellent radiation tumor response while only 48% of the negative reactors had an equivalent response. Almost twice as many strong DNCB reactors were alive and free of disease (NED) at six months, (70% versus 38% for non reactors). Similarly, the positive DNCB reactors had a median survival of 18 months compared to 10 months for the non reactors.